Glow plugs are used to aid starting of internal combustion engines such as diesel engines. Each of the glow plugs has a cylindrical housing and a heater for generating heat upon energization thereof. As the heater, there can be used a ceramic heater having a conductive ceramic heating element or a sheath heater having a heating coil.
The housing includes a thread portion for mounting to the internal combustion engine, a tool engagement portion for engaging with a tool during mounting of the glow plug onto the internal combustion engine and a press contact portion for, upon screwing the thread portion into a mounting hole of the internal combustion engine, making press contact with a plug seat surface of the internal combustion engine so as to secure gastightness of a combustion chamber of the internal combustion engine.
The housing also includes a cylindrical front body portion located between the thread portion and the press contact portion. In the state where the glow plug is mounted onto the internal combustion engine, a compressive force (axial force) is exerted on the front body portion of the housing in an axis direction of the glow plug. Further, the housing may include a cylindrical rear body portion located between the thread portion and the tool engagement portion. A force (torsion stress) is exerted on the rear body portion of the housing in a circumferential direction of the housing during mounting of the glow plug onto the internal combustion engine by the tool (see e.g. Patent Document 1).